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-   -   LM4780 Kit and Transformer Selection (http://www.diyaudio.com/forums/chip-amps/102491-lm4780-kit-transformer-selection.html)

bacworth 25th May 2007 03:14 PM

LM4780 Kit and Transformer Selection
 
I am looking at buying the LM4780 kit from here: http://www.audiosector.com/lm4780.shtml

Firstly, does anyone have expereince with this kit. Are ther others avaible using the LM4780 chip that you can recommend?

After trawling through numerous posts on this chip to determine which is the best transformer to drive 4ohms loads, I am more confused now than when I started. So which of the following would be the best:

30-0-30
25-0-25
18-0-18

I want to use two tranformers and have a choice of either 160VA or 300VA in the above volatges.

Also if I wanted to upgrade the LM4780 kit where is the best place to start?

twitchie 25th May 2007 04:59 PM

I'm sure Peter will chime in himself, but I have one of Peter's 4780 kits built and running. It is a very nice kit and if you want upgrades, Peter has upgrades as well (blackgate caps and riken and caddock resistors).

For a 4-ohm load, you could parallel the 2 channels (per chip) and use the 25V transformer, or play it safe and get the 18V. I use 22V on my LM4780.

Nordic 25th May 2007 07:11 PM

The 18V should give about 25.5V DC after rectification, but with realistic diode drops it will be a little less than that... somewhere between 22 and 23V which is safe for 4 ohm loads.

You could get away with 160va per channel, or use 1 300VA transformer... or better yet 2...less chances of gruond loops and hums with two seperately powered channels... however this beeing a stereo chip, I guess you are forced to use one transformer...

Mayday 25th May 2007 10:17 PM

I've built the "BridgeClone" from here and I'm very satisfied with it. Using one 500VA 2x24V~ trafo and 60000uF of elektrolytes.
Of course I'm using a softstart for it.

bacworth 25th May 2007 11:21 PM

If I use the chips in parallel what kind of power can I expect using an 18v or a 25v transformer? Any ideas on heatsinking requirements for each of these for this configuration?

Mayday 25th May 2007 11:29 PM

In bridged mode the LM4780 gives close to 120W and gets really hot!
This is what I used
http://bildarkiv.hififorum.nu/mayday...C_chassi_2.jpg

http://bildarkiv.hififorum.nu/mayday...C_chassi_4.jpg

And that's a 500VA hunk of a trafo.

Maybe a tad bit overkill, but not much.
BTW I'll be using the same kind of heatsinks for my Mini-Aleph, 10W Class A.

Peter Daniel 26th May 2007 03:27 PM

2 Attachment(s)
Quote:

Originally posted by bacworth
If I use the chips in parallel what kind of power can I expect using an 18v or a 25v transformer? Any ideas on heatsinking requirements for each of these for this configuration?
According to graph below (taken from http://cache.national.com/ds/LM/LM4780.pdf ) you can expect 60W with 18v and 120W with 25v transformers (parallel mode, 4 ohm load). The actual power will also depend on PS regulation properties, with lower wattage transformers (and lower capacitance), the voltage may drop substantially and it will affect power output as well.

Peter Daniel 26th May 2007 03:35 PM

Re: LM4780 Kit and Transformer Selection
 
Quote:

Originally posted by bacworth
Also if I wanted to upgrade the LM4780 kit where is the best place to start?
Probably transformer and capacitors in PS, you may also consider using better resistors.

bacworth 27th May 2007 02:33 AM

2 Attachment(s)
Still a little confused, for an 18v tansformer (approx 25.4v at the rails) isn't the attached spreadsheet correct? Or should Vcc/Vee be the transformer voltage not the rail voltage?

There are conflicting posts floating around here.

meche 30th May 2007 09:42 PM

bacworth, where did you get this spreadsheet?


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